1. Field of the Invention
The present invention relates to an ESD (Electrostatic Discharge) analysis device and an ESD analysis program which are used for designing a semiconductor device, and a method of designing a semiconductor device.
2. Description of Related Art
It is known that ESD (Electrostatic Discharge) analysis is carried out in designing a semiconductor device. For example, Japanese Laid-Open Patent Application JP-P 2005-196468 A (corresponding to US2005146380 A1) discloses an apparatus and a program for analysis of electrostatic discharge of a semiconductor integrated circuit. The apparatus for the analysis of the electrostatic discharge of the semiconductor integrated circuit has a resistor network generation portion, a protection circuit generation portion and an analysis portion. The resistor network generation portion generates a resistor network as a circuit equivalent to power supply wirings based on wiring pitches, wiring widths and sheet resistances of the power supply wirings in a logic cell area of the semiconductor integrated circuit. The protection circuit generation portion generates an electrostatic discharge protection network in which protection elements and pads placed in I/O cell areas of the semiconductor integrated circuit are connected to the resistor network. The analysis portion calculates voltages between the pads when currents equivalent to electrostatic discharges flow between the pads.
An ESD analysis flow according to this related art is as follows. First, modeling of the power supply wiring of LSI is executed with resistors to generate a grid-like power supply model (resistor network). Next, an I/O cell model including ESD protection elements and pads is connected to the power supply model to generate an ESD protection circuit model (electrostatic discharge protection circuit network). A shortest path between two pads in the ESD protection circuit model is searched and potential difference between the pads is calculated.
In recent years, power saving has been getting much important in the LSI. For this reason, for example, a technique that an unused circuit region is put into an OFF state or a technique that supplied voltages is decreased are adopted to a semiconductor chip. In these cases, a circuit in the semiconductor chip is divided into a plurality of circuit regions. Each circuit region belongs to one of a plurality of power supply systems. Each power supply system is controlled independently. Such semiconductor chip has a problem that cells (border cells) inputting and outputting signals to and from one circuit region in one power supply system to another circuit region in another power supply system are easy to be influenced by ESD. For example, when one CMOS inverter is used as a border cell in one power supply system at an output side and another CMOS inverter is used as a border cell in another power supply system at an input side, a through current can pass from the CMOS inverter at the output side to the CMOS inverter at the input side associated with application of ESD. In this case, a gate oxide film of the CMOS inverter at the input side may cause dielectric breakdown. Thus, there is a demand for the technique capable of appropriately analyzing influence of ESD on the border cells.
As for another related art, Japanese Laid-Open Patent Application JP-P 2004-282058 A (corresponding to US2006103421 A1) discloses a semiconductor integrated circuit device and a method of designing the same. The semiconductor integrated circuit device includes a first power supply wiring, a second power supply wiring, a first circuit portion power-supplied between the first and second power supply wirings, a third power supply wiring, a fourth power supply wiring, a second circuit portion power-supplied between the third and fourth power supply wirings, a first interface circuit formed in the first circuit, and a second interface circuit formed in the second circuit for inputting and outputting signals from and to the first interface circuit. The second power supply wiring is connected to the fourth power supply wiring. The second interface circuit is placed in the vicinity of the first interface circuit.
We have now discovered following facts. According to the ESD analysis disclosed in Japanese Laid-Open Patent Application JP-P 2005-196468 A, in a case of a single power source, the ESD protection circuit should be designed so that voltage applied to the power supply protection elements may be a predetermined voltage or smaller at the time of application of ESD. In this case, since voltage which is equal to or larger than the voltage applied to the power supply protection elements is not applied to cells, no problem occurs. However, when the ESD analysis of this related art is applied to a semiconductor device having a plurality of power supply systems, the following problems may occur.
FIG. 1 is a circuit model diagram for describing problems caused when the ESD analysis of the related art is applied to a semiconductor device having a plurality of power supply systems. The semiconductor device 101 includes a core region 105 and an I/O region 104 surrounding the core region 105. Each of the I/O region 104 and the core region 105 is divided into a circuit region 102 of a first power supply system (VDD1) and a circuit region 103 of a second power supply system (VDD2). The circuit region 102 belongs to the first power supply system and a first power source applies a voltage VDD1 to the circuit region 102. The circuit region 103 belongs to the second power supply system and a second power source applies a voltage VDD2 to the circuit region 103. The I/O region 104 of the circuit region 102 includes a plurality of pads 111 (including a pad 111A) and a plurality of ESD protection elements 112 (including pads 112-1, 112-2). The I/O region 104 of the circuit region 103 includes a plurality of pads 121 (including a pad 121B) and a plurality of ESD protection elements 122 (including pad 122-1, 122-2). The core region 105 in the circuit region 102 is connected to the core region 105 in the circuit region 103 with border cells 131 so as to be able to transmit data to each other. For example, one of the border cells 131 (border cells 131out) belongs to the circuit region 102 and the other of the border cells 131 (border cells 131in) belongs to the circuit region 103.
When ESD is applied between the circuit region 102 and the circuit region 103 of different power supply systems as shown in FIG. 1, a potential difference between the sums of voltages applied to two power supply protection elements may occur between border cells 131. In FIG. 1, consider that ESD is applied between the pad 111A as an ESD current inlet and the pad 121B as an ESD current outlet. The circuit region 102 of the first power supply system (VDD1) includes ESD protection elements 112-1, 112-2 and the circuit region 103 of the second power supply system (VDD2) includes ESD protection elements 122-1, 122-2. Thus, there are four routes possible to pass current. The first one is a route from the pad 111A through the ESD protection element 112-1 and then the ESD protection element 122-1 to the pad 121B. The second one is a route from the pad 111A through the ESD protection element 112-2 and then the ESD protection element 122-1 to the pad 121B. The third one is a route from the pad 111A through the ESD protection element 112-1 and then the ESD protection element 122-2 to the pad 121B. The final one is a route from the pad 111A through the ESD protection element 112-2 and then the ESD protection element 122-2 to the pad 121B. Thus, a potential difference occurs between a terminal of the ESD protection element 112-1 in the first power supply system (VDD1) side and a terminal of the ESD protection element 112-2 in the VDD1 power supply systems. As a result, currents also pass through wirings between the ESD protection element 112-1 and the ESD protection element 112-2 in the circuit region 102, thereby generating a potential difference. Consequently, the potential of the terminals of the border cell 131out in the first power supply system side is determined depending on a wiring resistance between the ESD protection element 112-1 and the border cell 131out and a wiring resistance between the ESD protection element 112-2 and the border cell 131out. That is, the potential cannot be obtained unless a circuit simulation is performed in consideration of wiring resistance between the ESD protection element 112-1 and the border cell 131out and wiring resistance between the ESD protection element 112-2 and the border cell 131out. This also applies to a potential of terminals of the border cell 131in in the second power supply system (VDD2) side. In other words, if the ESD analysis of the related art is applied to the semiconductor device including a plurality of power supply systems, a suitable ESD analysis cannot be achieved.
Furthermore, when the ESD analysis disclosed in Japanese Laid-Open Patent Application JP-P 2005-196468 A is applied to the semiconductor device having a plurality of power supply systems, the following problems may occur. First, in the border cells transferring signals between different power supply systems, a potential difference which determines whether or not gate breakdown (dielectric breakdown of gate oxide film) occurs cannot be explicitly analyzed. This is due to that, since information on the border cells across different power supply systems and their potential of the border cells is not outputted, the potential difference between border cells cannot be outputted.
Second, in the ESD analysis of the related art, only information on a shortest path among pads is outputted, and even if there is a border cell which possibly causes gate breakdown in the other paths, the border cell cannot be found. This is due to that, since an ESD check is performed by searching the shortest path between two pads, a dangerous point for the ESD in paths other than the shortest path cannot be considered.
Third, since a reference with respect to a potential difference between pads needs to be set, limitation may become stricter than necessary and flexibility in design may be lowered. This is due to that, in spite that the potential difference between the border cells determines whether or not gate breakdown occurs actually, the reference is set based on a potential difference between pads in a shortest path.
In designing a semiconductor device having a plurality of power supply systems, there is a demand for a technique capable of carrying out an ESD analysis more accurately. In designing a semiconductor device having a plurality of power supply systems, there is a demand for a technique capable of relaxing ESD standards and improving flexibility in design of the semiconductor device.